Dsc Lab Report 2j3e5n

Discussion: All thermograms obtained showed endothermic reactions of materials changing from a solid to a liquid state (melting). This suggests that all the material were crystalline as the melting point is the temperature at which the crystal lattice breaks up. Amorphous materials do not have a melting point because they do not have a crystal lattice to break1. The maximum points of the peaks correspond to the melting points of the materials. The beginning and end points of the peaks were determined and connected. The area of the peak was determined and this in turn was used to calculate the enthalpy of the reaction/the latent heat of melting for each material. There is a very small margin for error here as to what is perceived to be the beginning and end points. The thermograms obtained for magnesium stearate (MS) and ground magnesium stearate (GMS) had evaporation peaks. This suggests that this sample of magnesium stearate has water bound to it and the water was driven off as the temperature was increased thus generating the evaporation peak. During the process of crystallization, water molecules were trapped within the lattice giving a hydrate1. The peak obtained for GMS showed a shoulder (please refer to thermogram). This occurs due to an improvement in heat transfer. The particles are more uniform in size and there is less air between the particles allowing for heat to be conducted more efficiently. When comparing the GMS and MS thermograms, it can be seen that the GMS has a higher melting point but a lower latent heat of melting than MS. This is the same with methylcellulose (M50) and microcrystalline cellulose (MCC), with M50 having a higher melting point but lower latent heat of melting than MCC. Grinding (GMS) and the addition of the methyl group (M50) have resulted in the formation of and polymorphs ie. some molecules have formed crystals with a different packing pattern. There are two types of polymorphism; monotropic and enantropic polymorphism. Monotropic polymorphism is when only one polymorph is stable and any other polymorph that is formed will eventually convert to the stable form. Enantropic polymorphism is when under different conditions (temperature and pressure) the material can reversibly transform between alternative stable forms1. GMS and M50 appear to exhibit enantropic polymorphism because changes in the latent heat of melting occur under changing temperature. The more stable polymorph under these conditions seems to be one that has a higher melting point thus the melting point for GMS and M50 are higher than that of MS and MCC, respectively. The most stable polymorph also appears to have a more tightly packed arrangement giving better heat conduction and therefore lowering the latent heat of melting of GMS and M50 when compared to MS and MCC, respectively. The M50 and MCC peaks are quite broad suggesting that the samples are not pure and have a lot of impurities present. The MS and GMS peaks are of medium

width suggesting that there are some impurities present. A pure sample would exhibit a narrow peak.

Thermal analysis techniques. 1. Melting point machine: is used to determine the melting point of materials. The melting point machine is set at a plateau temperature which is lower than the melting point of the substance. The sample is place inside the machine and the temperature is increased by regular increments until the sample is seen to melt. There is a great margin for error as this method relies on the visual analysis of an individual and dependent on that person’s eyesight and when they perceive the sample to start melting. Also, the way the sample is packed into the capillary tube can result in errors. If the sample is loosely packed then there would be an error in the determination of the melting point because the air between the particles would decrease the efficiency with which heat is conducted throughout the sample. 2. Heating and Cooling Curves: the sample is heated at a uniform rate and its temperature is recorded automatically with a thermocouple placed in the sample. The sample temperature is related to an external isothermal standard and is recorded against time or the furnace temperature. During an endothermic phase transition, such as melting, heat is absorbed and the sample temperature remains constant at the melting point until the transition occurs. Such heating curves give only qualitative information about the changes in the sample. Their use is generally limited to the determination of transition temperatures, although some information on the purity of the sample may be obtained from the shape of the curve2. 3. Thermogravimetry: the mass of a sample in a controlled atmosphere is recorded continuously as a function of time or temperature as the temperature of the sample is increased (usually linearly with time). A plot of mass or mass percent as a function of time is called a thermogram or a thermal decomposition curve. The information provided by this method is limited because here a temperature variation must bring about a change in the mass of the analyte. Therefore, thermogravimetric methods are largely limited to decomposition and oxidation reactions and to such physical processes as vaporisation, sublimation and desorption4. 4. Differential Thermal Analysis (DTA): the difference in temperature between a substance and a reference material is measured as a function of temperature while the substance and reference material are subjected to a controlled temperature program. The temperature program involves heating the sample and reference material in such a way that the temperature of the sample increases linearly with time. The temperature difference between the sample and reference temperatures is monitored

and plotted against sample temperature to give a differential thermogram4. These temperature differences may lead to heat fluxes, the course of which has a considerable impact on the DTA curve 3. This method provides a simple way of determining the melting, boiling and decomposition points of organic compounds as well as an indication of sample purity4. 5. Differential Scanning Calorimetry (DSC): differences in heat flow into a substance and a reference are measured as a function of sample temperature while the two are subjected to a controlled temperature program. The heating rate required to keep the sample and reference at the same temperature is plotted against time or temperature 4. DSC instruments measure the energy change in the sample directly, not as a temperature change and they are consequently more suitable than DTA for quantitative measurements of heats of reaction and transition, specific heats, etc. DSC measurements may be made isothermally or at very low heating rates without loss of sensitivity. DSC is also used to estimate purity, measure transition temperatures and identify materials. DSC can be used to analyse volatile samples2. To conclude, DSC is a superior thermal analysis method in that it can quantify the enthalpy of the reaction as well as carry out all of the applications of DTA. References: 1. Aulton, M.E., (2007), Aulton’s Pharmaceutics: The Design and Manufacture of Medicines, 3rd ed, Churchill Livingstone, pg 13,111-115. 2. Daniels, T., (1973), Thermal Analysis, Kogan Page Limited, pg 73, 74, 122, 132. 3. Schwedt, G., (2005), The Essential Guide to Analytical Chemistry, Wiley, pg 76, 80. 4. Skoog, D.A., Holler, F.J. and Nieman, T.A., (1998), Principals of Instrumental Analysis 5th ed., Brooks/Cole, pg 798-808.